my daughters are now 10 and almost 14. We are so happy to say that the 13 3/4 yr old is ALMOST touching the bottom line of the growth curve for her age! She is almost 75 lbs, which is the average weight of a 10 yr old. The younger one is a bit heftier at the 25th percentile for her age, weighing in at 64 lbs. She's bulked up this past year.
both of them were diagnosed failure to thrive, but the new term is failure to gain weight since they ARE thriving (happy, able to play, good skin tone, etc.)
At this age your son should be eating all four food groups, and meat is a good bulk-er, but nuts or hummus would work for that too.
With our first we freaked out. Now granted at 18 months the endocronologist (sp?) didn't tell us how valid our freak out was til we went back 4 weeks later and he told us that she had been in the latter stages of starvation. Apparently she no longer had fat pads on her toes or fingers at that point. Her hair also had gotten brittle. I'm still angry that he didn't admit her since she literally could have just died on us!
She was tested for celiac disease, CF and many other things. The celiac was inconclusive so we had a biopsy done. It was clear but at 10 we repeated the blood test since when it was done as a toddler she had not eaten grains (on her own choice) very often. She did the barium swallow and so many blood tests. Some came back ok, others not, but then the next test would be that thing ok and a different thing wrong. I hope that you have had those tests done because sometimes it IS due to a real issue. Finally the diagnosis was a super high metabolism, and we had her eating the fattest foods and drinking a "weight gain cocktail" which is 1 cup milk, 1 cup heavy cream and 1 packet Carnation Instant Breakfast.
I must say though that I think she was not born with a hunger mechanism. She didn't ASK for food till she was a few days shy of 4 years old. The thing that changed it for her was taking milk out of her diet. This also made her finally sleep through the night.
When our second daughter came around we weren't as freaked out. Even though she was almost oz for oz what the older one had been at each age she was MUCH shorter so she looked just fine, or at least not as bad. The docs thought we were crazy when we said she was so chunky, lol. She has always been around the 10 to 25th percentile but her height was not the 75th that her big sister was but more like the 50th.
My older daughter is having issues now that she is almost 14 and so skinny. Finding cool clothes that fit is hard. When she went to 6th grade and still was in size 13 kids shoes it was really hard for her to get non kiddy shoes. But overall it has not been a major problem.